Recent work in our laboratory has resulted in the successful isolation and identification of platelet glycoprotein IV (GPIV, CD36, GPIIIb) as a membrane receptor for thrombospondin (TSP). This application proposes study of TSP and its interaction with cellular receptors as a paradigm for cell adhesion processes relevant to platelet aggregation, cell adhesion and tumor metastasis. TSP is a 450 kD trimeric glycoprotein that is a major platelet alpha granule constituent and is expressed on the activated platelet surface where it supports irreversible platelet aggregation. TSP is synthesized by a variety of cell types and is incorporated into the matrix of growing cells where it appears to regulate cell growth, differentiation, and adhesion. Defining the interactions of TSP with its cellular receptors is critical to an understanding of the function of TSP. In preliminary studies, we have defined a domain within TSP that is responsible for its interaction with the cellular receptor GPIV. The therapeutic implications of a peptide that might interfere with TSP's role in platelet aggregation of tumor metastasis are profound. In the proposed experiments, an examination of TSP domains that participate in cellular TSP interactions will be pursued. Specifically, the experiments outlined in this proposal will address the following: 1.Identification of the cellular binding domains of TSP. The domain(s) of TSP that participate in cell binding will be defined in studies measuring the binding of TSP and synthetic TSP peptides. 2.Identification of the Iigand (TSP) binding domains of GPIV. Crosslinking studies will be used to identify the domain(s) of the receptor that interact with TSP. 3.Investigation of the role of TSP and GPIV in tumor cell biology. Studies on the adhesion of tumor cells to TSP coated surfaces will be performed to clarify the role of this adhesive system in tumor cell metastasis.